This invention relates generally to electrical circuits for operating arc discharge flash lamps and, more particularly, to an improved circuit arrangement for triggering a flash lamp which is directly coupled to an alternating current (AC) source.
Flash lamps of the type referred to herein generally comprise two spaced apart electrodes within an hermetically sealed glass envelope having a rare gas fill, typically xenon, at a subatmospheric pressure. In typical prior art operating circuits, such lamps are connected across a large energy storage device, such as a bank of capacitors, charged to a substantial potential, but insufficient to ionize the xenon gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the storage device through the flash lamp, which emits a burst of intense light. In many cases the pulse voltage is applied between an external trigger electrode, such as a wire wrapped around the envelope, and one of the electrodes; this is referred to as shunt triggering. However, in other cases an external wire is not feasible since it may result in an undesirable arcing between the trigger wire and a proximate lamp reflector, or else the high potential applied to the external trigger wire might be hazardous to operating personnel. In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes, a technique referred to as injection triggering. Usually the voltage required is about 30 to 50 percent higher than that required to trigger the same lamp with an external trigger wire, and the trigger transformer secondary must carry the full lamp current.
Such flash lamps are employed in a variety of applications; for example, flash photography; reprographic machines; laser excitation; and warning flashers on airplanes, towers, road barriers, marine equipment and tower mounted approach lighting systems for airport runways. Typical prior art power supplies pose serious disadvantages for a number of these applications, however, as the required energy storage devices, such as large banks of capacitors, tend to be bulky, heavy and expensive, as are required step-up transformers. This is particularly apparent in endeavors to provide compact, low cost photographic flash lamps, or light weight runway flashers for mounting on frangible towers. Accordingly, it is particularly desirable to find a means for eliminating the large energy storage devices in flash lamp power supplies. In pursuit of this end, it has been observed that much higher than average short duration currents are routinely drawn from AC power lines; for example, compressor motor starting transients (locked rotor currents) are four to seven times their running currents. Metal fuses, another example, can handle peak half cycle currents 10 or more times their continuous ratings. Hence, in order to overcome the aforementioned disadvantages, it has been proposed to take advantage of this high transient current capacity of conventional 120 volt, 60 Hertz AC power sources to draw controlled pulses of high current to operate flash lamps. Three U.S. patents that describe the direct coupling of flash lamps to an AC source are U.S. Pat. Nos. 3,497,768 Mathisen, 3,745,896 Sperti et al (FIGS. 20-25 and col. 14 on), and 3,896,396 Whitehouse et al. In Mathisen, a silicon controlled rectifier (SCR) is connected in series with a xenon flash lamp across the secondary winding of a step-up transformer, the primary of which is connected to a conventional 60 Hertz, 120 volt AC source. A storage capacitor normally charged from the AC source is coupled via a pulse transformer to the trigger electrode on the lamp. When the lamp is to be energized, a switch operated trigger circuit places the SCR in a conductive state to connect the lamp directly across the AC source (transformer secondary) during a properly poled half cycle of the input voltage, and the storage capacitor also discharges through the SCR and pulse transformer winding to apply a high voltage starting pulse to the trigger electrode of the lamp. In this manner, the Mathisen lamp is energized for approximately one-half cycle of the AC waveform to provide a short duration, high intensity source of radiation.
In Sperti et al, a flash lamp is connected directly across a conventional AC source through a series resistor which provides overcurrent protection. The trigger circuit of FIG. 20 includes a half-wave rectifier connected across the AC source, a pair of storage capacitors, and an interrupter, such as a magnetic reed switch, which is connected to the trigger electrode of the lamp. In operation, one of the capacitors is charged by the AC source, then, when a trigger switch is closed, the charge is transferred to the second capacitor to provide a source of DC to the interrupter. This DC is transformed by the interrupter into a pulsating high voltage current which is applied to the trigger electrode to ionize the lamp. The interrupting frequency is about 300 Hertz, and flash duration, which is dependent upon the dissipation of the charge on the second capacitor, may extend over more than one half of a power cycle. In the variation of FIG. 21 of Sperti et al, there is no second storage capacitor, and closure of the trigger switch turns on an SCR through which the interrupter is energized by the charge on the initial storage capacitor. The variation of FIG. 22 of Sperti et al, employs a capacitor discharge to turn on the SCR. In FIG. 23 of Sperti et al, closure of the trigger switch provides a measured power pulse from a capacitor which momentarily energizes a relay which actuates a switch for connecting the AC source across the primary of a transformer having a 2000 volt output. A spark gap is connected across the secondary of this transformer, and connected in parallel with the spark gap is a storage capacitor in series with the primary of a radio frequency transformer having a secondary connected to the trigger electrode on the lamp. Hence, when the AC source is connected to the 2000 volt transformer, the spark gap breaks down, thereby causing a short circuit across the transformer and discharging the storage capacitor through the arc. This, in turn, causes a large voltage to appear across the secondary of the radio frequency transformer, whereupon the lamp is triggered to flash. The 2000 volt transformer produces spark gap break down on both halves of the AC cycle; hence, a pulsating radio frequency trigger voltage is produced at 1/120 second intervals. FIG. 24 is similar to FIG. 23 except that closure of the trigger switch turns on an SCR which energizes the relay. FIG. 25 is similar to FIG. 24, except that a capacitor discharge is employed to turn on the SCR.
In Whitehouse et al, a flash lamp is connected directly across an AC source through a series diode. One embodiment shows a flash lamp being excited from two phases of a three-phase Y-connected AC source so as to permit lengthening of the flash lamp pulses over that possible with a single phase system. FIG. 3 of Whitehouse et al employs a pair of capacitors across the lamp in connection with a capacitor charger to add to the current surge through the lamp during initial firing. The charger is described as including a transformer energized by a third phase of the AC source and a rectifying diode. The trigger circuitry is shown in FIG. 4 of the patent and includes a logic circuit which senses the AC voltage across one phase of the source to produce a narrow pulse at the desired angle. The logic circuit comprises a full wave rectifier connected between the AC source and the input of a monostable multivibrator which produces an output pulse at the phase angle of each cycle selected by a firing angle adjustment (not described). The monostable pulses are coupled to both a digital counter and one input of an AND gate. The digital counter functions to count out the desired number of pulses firing the flash lamp with each pulse, then counts out a pause between bursts of pulses. The number of pulses in a burst of pulses from the counter is set by external switches. The counter produces a binary 0 state when the desired number of pulses has been counted and applies this signal to a second input of the AND gate. The burst of pulses (e.g. three 60 Hertz pulses) at the output of the AND gate is coupled through a first pulse transformer to trigger an SCR into conduction. The SCR is connected in the primary circuit of a second pulse transformer coupled to the trigger electrode of the flash lamp. This primary circuit also includes a capacitor and a resistor connected to a +200 volt DC supply. When the SCR is switched on, this capacitor is coupled across the primary winding of the second transformer, and current through the loop will be a half sine wave pulse. This results since the capacitor and transformer inductance form a resonant circuit, and the current cannot reverse through the SCR. The pulse is then coupled through the second transformer to ignite the lamp. In a specific embodiment, three pulses are counted out each burst to flash the lamp three times; then after counting out a pause, the same burst of three pulses will be repeated.
Although avoiding the need for large storage capacitors, direct-line-coupled flash lamp circuits can exhibit their own peculiar problem areas. For example, due to the rather arbitrary relationship between the time of initiating switch activation and the phase of the AC source waveform, it is possible to have significant variations in light intensity from one flash to another. In fact, lack of precise synchronization may result in occasional failure of the lamp to flash when requested.